This document is not restricted to specific software and hardware
versions.

The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.

HSRP is often used to improve resiliency in networks, but this can
cause a decrease in network efficiency. The example in this document has two
paths from the host network to the server network. For redundancy, HSRP runs
between R1 and R2, either of which can become the active router and take
"ownership" of the HSRP virtual IP address. The second router becomes the
standby router, and only becomes the active router if the current active router
goes down. For more information about active and standby routers, refer to
How
to Use the standby preempt and standby
track Commands.

The hosts' default gateway address is assigned as their HSRP virtual IP
address. When the hosts need to send packets to the server network, they send
them to their default gateway, or to whichever router is active. Because only
one router is active, packets from the hosts to the servers traverse only one
of the two available paths.

Note: Dependent upon on how you configure R3, packets that return from the
servers to the hosts might or might not utilize both return paths. Also,
packets that return from the servers to the hosts do not need to go through the
active router.

In order to utilize both paths from the host network to the server
network, you can configure Multigroup HSRP (MHSRP) between R1 and R2.
Essentially, R1 is configured with two HSRP groups (for example, group 1 and
group 2) and R2 is also configured with the same HSRP groups. For group 1, R1
is the active router and R2 is the standby router. For group 2, R2 is the
active router and R1 is the standby router. Then you configure half of the
hosts' default gateways with the HSRP group 1 virtual IP address, and the other
half of the hosts' default gateways with the HSRP group 2 virtual IP address.

Notice from the configurations that when the two routers first begin to
run HSRP, R1 has a default priority of 100 for group 1 and a priority of 95 for
group 2. R2 has a default priority of 100 for group 2 and a priority of 95 for
group 1. Therefore, R1 is the active router for group 1 and R2 is the active
router for group 2. This example shows that you can accomplish load-sharing
with MHSRP. However, you need to use HSRP priority and preempt in order to
accomplish this. HSRP has no effect on return traffic. The path taken by the
return traffic depends on the routing protocol configured on the router.

Note: When the standby
priority value and
standby
preempt commands are configured, it is mandatory to
explicitly mention the group number. If not mentioned, then the value is 0 by
default. The default group number is 0.

Several Ethernet (Lance and QUICC) controllers in low-end products can
only have a single unicast Media Access Control (MAC) address in their address
filter. These platforms only permit a single HSRP group, and they change the
interface address to the HSRP virtual MAC address when the group becomes
active. Load sharing on platforms with this limitation is not possible with
HSRP. The use-bia command was introduced to work
around issues that occur when you run HSRP on the low-end products as
mentioned. For example, if you run HSRP and DECnet on the same interfaces,
problems occur because DECnet and HSRP try to modify the MAC address. With the
use-bia command, you can configure HSRP to use the
MAC address created by the DECnet process. However, you should be aware that
there are some disadvantages if you use the use-bia
command, such as:

When a router becomes active the virtual IP address is moved to a
different MAC address. The newly active router sends a gratuitous Address
Resolution Protocol (ARP) response, but not all host implementations handle the
gratuitous ARP correctly.

The use-bia configuration breaks proxy
ARP. A standby router cannot cover for the lost proxy ARP database of the
failed router.